The present invention relates generally to batch ion implantation systems, and more particularly to an improved system, apparatus, and method for reducing particle generation and surface adhesion forces in a batch ion implanter, while maintaining a high heat transfer coefficient.
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in xe2x80x9cn typexe2x80x9d extrinsic material wafers, whereas if xe2x80x9cp typexe2x80x9d extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
Ion implanters may be separated into two different categories. The first category includes serial ion implanters, in which semiconductor wafers or other workpieces are completely implanted with ions in serial fashion. This type of implanter includes a single workpiece pad adapted to hold or support the workpiece being implanted. The second category of ion implanters includes batch implanters, wherein a plurality of wafers or other workpieces may be implanted with ions in a single batch. The workpieces being implanted are mounted on individual workpiece pads in a rotatable process disk. The workpiece pads are typically located on individual pedestals extending outward from a center portion of the process disk at a slight angle so as to use centrifugal force to keep the workpieces seated in the pads as the process disk is rotated in a controlled fashion via a drive motor. The ion source is located so as to present ions along a beam path offset from the rotational axis of the process disk, and thereby to implant ions onto the workpieces as they rotate into the beam path. This method of ion implantation is sometimes referred to as spinning disk ion implantation.
As ions are implanted in the workpieces in a batch ion implantation process, heat is generated within each workpiece, which may cause workpiece damage or other deleterious effects if the heat is not removed from the workpiece. Conventional batch ion implantation systems and apparatus remove heat from the process disk and pedestals onto which the workpieces are mounted using internal passages through which cooling fluid such as water is circulated. Heat is removed from the workpieces to the process disk through workpiece pads comprising vulcanized rubber or RTV on which the workpieces are seated. Therefore, one function of the workpiece pads are to transfer heat from each workpiece to the process disk. Another function of the workpiece pads are to provide a tacky surface whereon each workpiece resides, whereby the workpiece can be sufficiently retained during the rotation of the process disk.
RTV workpiece pads are typically formed via a molding process, wherein liquid-state RTV material is cured after being applied to a surface of a mold which has been coated with a mold release agent. A conventional mold surface comprises a lapped metal plate, wherein a rough surface comprising substantially random peaks and valleys are formed as a result of lapping the metal plate with lapping compound. An imprint of the peaks and valleys are further transferred to the RTV workpiece pad via the molding process.
The peaks and valleys on a conventional workpiece pad formed via a lapped metal plate mold facilitate removal of the wafer from the workpiece pad after ion implantation. The rough surface on the workpiece pad is utilized to decrease a total contact area between the workpiece and the workpiece pad in order to reduce surface adhesion forces. During a conventional batch ion implantation process, for example, a workpiece such as a silicon wafer is placed on each RTV workpiece pad. The wafers are then rotated through an ion beam, whereby ions are implanted in the silicon wafers. Rotating the process disk results in an increased normal force on each workpiece pad caused by centrifugal force pushing the wafer onto the workpiece pad. This normal force compresses the RTV pad, thereby further increasing the surface contact area between the wafer and the RTV pad. This increase in surface contact area increases the heat transfer from the wafer to the pedestal, as well as increasing the surface adhesion of the wafer to the RTV pad.
One disadvantage of utilizing a lapped metal plate for the mold surface is that contaminants comprising both lapping compound and fine particles of metal created during the lapping process are typically present on the mold surface after the lapping process and cleaning thereafter. These contaminants can be transferred to the RTV pads during the molding process, and can further be transferred to the workpiece or wafer substrate when placed on the RTV pads for ion implantation. Furthermore, a mold release agent is typically utilized to prevent sticking of the RTV material to the mold surface. The mold release agent is transferred to the workpiece pad, and, if not thoroughly cleaned and removed from the workpiece pad after molding, can further be transferred to the workpiece or wafer substrate. Contaminants on the workpiece can cause many detrimental effects to the quality of the resulting ion implanted workpiece.
Another disadvantage of utilizing a workpiece pad formed via a lapped metal plate mold is poor dimensional uniformity and location of the peaks and valleys. Poor dimensional uniformity can create locations on the workpiece pad which have greater surface contact area than other locations on the workpiece pad, thus causing non-uniform surface adhesion and non-uniform heat transfer properties. Furthermore, structural dimensions of the peaks and valleys formed by utilizing the lapped metal plate mold are limited by the grain size of the lapping compound utilized during the formation of the mold. These disadvantages can cause areas of the workpiece pad which adhere non-uniformly to the wafer. Such non-uniformity of adhesion of the wafer to the workpiece pad is disadvantageous such that a removal of the wafer from the workpiece pad can cause either cracking of the wafer, or disintegration of part of the wafer or workpiece pad. Furthermore, a disintegration of the wafer or workpiece pad either contaminates the workpiece pad with remnants of the removed silicon wafer or contaminating the wafer with disintegrated parts of the workpiece pad, respectively.
Typical workpiece pads have heretofore achieved some level of heat sinking for removal of heat from workpieces, however, particles remaining from formation of the workpiece pads have been a source of contamination on the workpiece. Furthermore, workpiece pads heretofore have provided a significant amount of surface adhesion to the workpiece when placed on the workpiece pad. Therefore, improved workpiece pads are desirable, wherein the improved workpiece pads provide reduced contamination, as well as reduced adhesion forces, while still providing sufficient heat removal from the workpiece.
Consequently, there is an unresolved need for an improved batch ion implantation system and apparatus which eliminate or minimize the problems associated with conventional workpiece pads, and which provide for reduced particle contamination and surface adhesion to the workpieces, while maintaining a high heat transfer coefficient.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention is directed to a system and apparatus which provides reduced surface adhesion forces on workpieces in a batch ion implantation operation, while reducing particle generation and maintaining a high heat transfer coefficient associated therewith. More particularly, the invention provides a workpiece pad and method of forming the same, comprising an ordered array of micro-structures, wherein a workpiece residing on the workpiece pad can be implanted with ions in a rotating or spinning batch implanter process disk. The provision of the ordered array of micro-structures allows for low particle contamination of the workpiece, while also limiting surface adhesion forces between the workpiece and the workpiece pad. Additionally, the ordered array of micro-structures allows for adequate heat transfer from the workpiece to the process disk, which may be advantageously combined with a circulation of cooling fluid through passages in the process disk to remove heat therefrom.
According to one aspect of the present invention, there is provided an ion implantation system comprising a workpiece pad having an ordered array of micro-structures. The ion implantation system further comprises a process disk mounted on a shaft for rotation about an axis, wherein the process disk is adapted to support one or more pedestals thereon. The one or more pedestals extend laterally outward from a center potion of the process disk. The workpiece pad is further mounted on the distal end of the pedestal, whereon a workpiece can reside.
According to another exemplary aspect of the present invention, the process disk comprises a plurality of pedestals extending laterally outward from a center portion of the process disk, wherein the pedestals may each include a workpiece pad radially disposed from the axis and adapted to support a workpiece thereon. The pedestals may comprise a fluid feed port in the corresponding workpiece pad, wherein fluid communication between the shaft and the workpiece pads is established. The system may further comprise a drive, such as a motor, adapted to provide rotation of the shaft with respect to the housing, and a cooling fluid source adapted to provide fluid to the back sides of the workpiece pads in order to remove heat from the workpieces as ions are implanted thereon.
According to yet another aspect of the invention, a substantially resilient workpiece pad is provided for supporting at least one workpiece in an ion implantation system. The workpiece pad comprises an ordered array of micro-structures, whereon the workpiece may reside. According to one aspect, each of the micro-structures comprises a base portion facing the pedestal and a workpiece contact portion facing the workpiece. A first cross-sectional area is associated with the base portion and a second cross-sectional area is associated with the workpiece contact portion. According to one aspect, the first cross-sectional area is greater than the second cross-sectional area, thereby resulting in a micro-structure in the general shape of a pyramid. The workpiece pad, for example, comprises a substantially resilient material such as RTV silicone, wherein a normal force exerted on the workpiece contact portion of the workpiece pad causes the plurality of micro-structures to resiliently compress. This allows the centrifugal force resulting from rotation of the workpiece in the process disk to deflect the workpiece pad to provide engagement there between, thereby resulting in an increase in a contact surface area of the workpiece pad which is in contact with the workpiece. An increase in the contact surface area results in a higher thermal conductivity between the workpiece and the workpiece pad.
According to another aspect of the invention, there is provided a retaining apparatus for retaining the workpiece on the workpiece pad. The retaining apparatus may be employed in a batch ion implanter with a rotatable process disk having at least one workpiece pad adapted to support the workpiece. According to one exemplary aspect, the retaining apparatus comprises a ring-shape retaining member mounted on the pedestal adapted to constrain the workpiece proximate the peripheral edge of the workpiece pad. According to another aspect, the retaining apparatus comprises a plurality of tabs protruding from the pedestal proximate to the peripheral edge of the workpiece pad adapted to constrain the workpiece. According to yet another aspect, the plurality of tabs are moveable with respect to the workpiece pad. In accordance with yet another aspect, the retaining apparatus comprises both moveable and stationary tabs.
In accordance with another aspect of the present invention, there is provided a method of forming a workpiece mount for an ion implantation system. The method first comprises placing a substantially flexible mold comprising a surface having an ordered array of inverted micro-structures on a substantially rigid plate, wherein the surface having the ordered array of inverted micro-structures faces away from the rigid plate. A liquified workpiece pad material is then applied to the surface of the mold. A pedestal is then applied to the liquified workpiece pad material, and the liquified workpiece pad material is further cured. Alternatively, the liquified workpiece pad material is applied to the pedestal, whereby the liquified workpiece pad material on the pedestal is further applied to the surface of the mold and cured. According to one aspect of the present invention, the workpiece pad material is cured by an addition of pressure, thus pushing the rigid plate and pedestal toward one another. According to another aspect of the present invention, the liquified workpiece pad material is cured by an addition of heat. After the liquified workpiece pad material is cured, the rigid plate is removed. The mold is further removed from the now cured workpiece pad material, thus resulting in a pedestal comprising a bonded workpiece pad, wherein the workpiece pad comprises a transferred impression of the ordered array of inverted micro-structures, and wherein no mold release agent is required to remove the mold from the workpiece pad material.